The world that Tolkien created is so rich in lore that the books he wrote only scratch the surface of the animals and races that inhabit it. The Hobbit and The Lord of the Rings feature only a chosen few characters and places that inhabit Middle-earth, a vast land that is filled with mystery.
It would take volumes to cover all or even most of the critters and monsters that make up the population of Middle-earth, a continent that stretches from the Shire to Mordor and beyond. A few of the following mysterious animals, beasts, and spirits have appeared on screen, while others remain in obscurity even though they might have come close to the path of the One Ring.
8 Durin's Bane
A creature that made a lasting impression in one of the most awesome scenes from Peter Jackson's trilogy, the existence of the Balrog in Khazad-dum was a mystery until the Fellowship made a dramatic visual confirmation. This is much more than an ancient demon, although Gandalf's description is accurate, it barely touches the long and sordid history of the Balrogs.
The Balrog in The Fellowship of the Ring is unnamed other than the title from the line of kings he broke, but it's the last of what used to be a whole platoon of demonic beings created by Morgoth. How it was trapped in the Mines of Moria and for how long can only be guessed, only that the Dwarves inadvertently woke it up.
7 "Nameless Things"
These are creatures so mysterious that even Gandalf doesn't know what they're called. He mentions them when describing his fight with the Balrog to Aragorn, Legolas, and Gimli as part of how deep he and the monster fell beneath the mountain.
Fans of both the movies and literature debate exactly what these things are, with some pointing to the serpents of the Norse underworld and others citing the old, forgotten gods from the east. The identity, age, or even the names of these creatures, is one of the persistent mysteries of Tolkien lore.
6 Gollum
There are mysterious figures in Tolkien lore that are famous, but here's one who's a mystery in the same way most people are. Smeagol was just an average Hobbit-like creature living with his grandmother by the river until the One Ring found him. Even then, he remained a nobody for hundreds of years, flying completely under the radar of heroes and Wizards.
Gandalf, who acts as the expository character in both the books and the screen adaptations, describes Gollum in detail to Frodo early in the book. However, outside of the Baggins' home, nobody knows anything about Gollum until Gandalf makes his dramatic oral presentation at the Council of Elrond.
5 Fastitocalon
A poem written by a Hobbit about the perils of the water would have to be about a giant turtle. Fastitcolan was the last of the giant turtle-fish that used to live in what is now Middle-earth, and this song about him and his lost kin appears in the book, The Adventures of Tom Bombadil.
Legend has it that the great turtle-fish used to appear as islands, only to doom any sailors taking refuge when they re-submerged. Gamers can find them in The Lord of the Rings Online as Fel Turtles and Fastiticolan himself appear in the wilds of Rohan as the biggest one of all.
4 Shelob
One of the reasons that Gollum was able to lure Frodo into the caves of Shelob is because the Hobbit wasn't aware of her existence. Nobody was, except for a few who remembered the savagery of Ungoliant, the giant spider that helped Morgoth destroy the Two Trees.
Given that fact, it makes sense that Galadriel, one of the few beings old enough to remember Ungoliant, is the character that gives Frodo the Light of Eärendil. It's hinted that Shelob is a smaller version of the ancient monster that allied with Morgoth, but whether or not that means she's a diminished version of the ancient spider or merely a descendent is never clarified.
3 Carcharoth
The werewolves that guarded Morgoth's palace were second only to the Balrogs when it came to their cruelty and fighting prowess. Their chief was Carcharoth, and he was raised and trained by Morgoth to guard his mater against Huan, a wolfhound that served the Vala Oromë and protected Beren and Luthien during their adventures.
Carcharoth was the werewolf that bit off Beren's hand and swallowed the Silmaril the unfortunate hero was carrying. The creature went mad and ran all over Middle-earth, inspiring a hunt that was still famous in the days of the War of the Ring.
2 Tom Bombadil
Perhaps the best-known mysterious character in Middle-earth, even Tolkien couldn't seem to pin down what this person is, or even if they are a person. What really clinches it for this enigmatic being is his immunity to the power of the One Ring, which sets him above a simple forest hermit or even a Maia like Gandalf.
Few other characters generate as much speculation as Tom Bombadil, and the mystery revolves around the simplest of questions. Who is he, and why is he here? Neither he nor his partner, the river spirit Goldberry, have anything to do with the story of the One Ring and have no connection to the Wizards or the Hobbits.
1 Scatha The Worm
Smaug is the dragon that everyone in Middle-earth knows about, and it's known that Morgoth created them as part of his army, but how many survived and their ultimate fate are lost to the ages. One of the dragons that plagued Middle-earth, but remains a mystery, is Scatha.
Little is known of Scatha before he appeared in the Grey Mountains to ravage the Dwarves who lived there. The story of this fearsome creature and how it was slain foreshadows the saga of the Lonely Mountain, specifically, how the dragon's death created a political frackus over who would get the hoard. The Horn of Rohan was part of his treasure, and it was eventually given to Meriadoc Brandybuck by Queen Éowyn.